Numerous works take into account the preparation of cyclic polychlorophosphazenes, which is relatively easy to carry out owing to the tendency to cyclize of the inferior compounds of the chlorophosphazene series. Linear polychlorophosphazenes, however, have a clearly greater economic interest than cyclic polychlorophosphazenes due to their very extensive possibilities of use as materials having applications of the type of those of silicones, of plastic materials and of natural or synthetic elastomers, as flameproof and incombustible materials or additives that impart flameproof and incombustible properties to the materials and substances to which they are added, as coatings, especially tight coatings, as materials utilizable in the biomedical field, as fertilizers, or even lubricants. Besides, certain elastomers obtained by different substitutions on linear polychlorophosphazenes have been found to have an excellent behavior at low temperatures, the same as with respect to corrosive reagents. The main application of the polychlorophosphazenes is their use as starting materials for obtaining polyorganophosphazenes, which are polymers of remarkable properties.
A few processes for the preparation of linear polychlorophosphazenes have been proposed in the prior art. Specifically, L. G. Lund, N. L. Paddock, J. E. Proctor and H. J. Searle (J. Chem. Soc. London, 1960, p. 2542) have described the preparation of polychlorophosphazenes in accordance with the following reaction diagram: ##EQU1## by working in a solvent consisting of symmetrical tetrachloroethane. However, this diagram implies a large number of reaction stages, the yield of which is rarely quantitative, and the mode of operation is lengthy and difficult inasmuch as the net product in which it results is a mixture of cyclic compounds present at a rate of 90% and of linear compounds of the type PCl.sub.5 (PNCl.sub.2).sub.n wherein n does not esceed 20 in a 10% proportion.
To obtain longer linear polymers, the cyclic compounds are then treated with solvents for separating the trimer and the tetramer from which there is extracted, by means of adequate solvents, the pure (NPCl.sub.2).sub.3 that is subjected to a thermal polymerization under reduced pressure, at a temperature of 250.degree. C., for two days, to yield a linear polymer (PNCl.sub.2).sub.n with an optimal conversion rate of 70% (H. R. Allcock, R. L. Kugel, K. J. Valan-Inorg. Chem. 1966, 5, p. 1709).
Such an indirect method of synthesis requires a substantial number of operations that are rarely quantitative and requires the use of considerable amounts of expensive solvents, which renders prohibitive the cost of production and weighs heavily on the price of the polychlorophosphazenes obtained. In addition, this method only allows the preparation of polychlorophosphazenes of very long chain length on the order of about 15,000 (NPCl.sub.2) patterns and it cannot be controlled to allow the obtention of polychlorophosphazenes where the chain length can be determined at will and especially of linear polychlorophosphazenes of a short or medium chain that is, including, for instance, from 4 to 1000 or more patterns.
In French Pat. No. 79 24037 (publication No. 2,466,435) of Sept. 27, 1979, there was proposed a process for the direct preparation of linear polychlorophosphazenes of short or medium chain length controlled by heating the compound p-trichloro-N-dicholorophosphoryl monophosphazene (rough formula P.sub.2 NOCl.sub.5), the polychlorophosphazenes obtained having a dichlorophosphoryl terminal group.
Likewise known is the p-trichloro N-dichlorothiophosphoryl monophosphazene (rough formula P.sub.2 NSCl.sub.5), the same as the dimer and trimer derived therefrom, the latter being obtained by indirect methods of synthesis. M. Becke-Goehring and W. Lehr (Z. Anorg. Allgem. Chem. 1963, 325, pp. 287 to 301) have thus prepared the dichlorothiophosphorylpentachlorodiphosphazene Cl.sub.2 (S)P --N=PCl.sub.2 --.sub.2 Cl by reacting H.sub.2 S on the ionic compound (Cl.sub.3 =N--PCl.sub.2 =NPCl.sub.3)+PCl.sub.6 -- obtained by the reaction of PCl.sub.5 with NH.sub.4 Cl in a solvent of a weak dielectric constant; but these authors have not been able to obtain superior homologues wherein the number of recurring --N=PCl.sub.2 -- units is more than 2 due to the formation of cyclic polychlorophosphazenes by pursuit of the reaction of the ionic compound with NH.sub.4 Cl. H. W. Roesky (Chem. Ber. 1972, 105 (4) pp. 1439 to 1445) has likewise proposed to synthesize the first members of the series of linear thiophosphorylchlorophosphazenes by gradually lengthening the P=N--P chain by successively causing to react the thiophosphorylchlorophosphazenes of short chain with the hexamethyl disilazane and PCl.sub.5. But he has been unable to go beyond the triphosphazene of formula Cl.sub.2 (S)P --N=PCl.sub.2 --.sub.3 Cl.
The dichlorothiophosphoryl polychlorophosphazenes obtained by either of the above two indirect methods of synthesis can hardly be industrially applied because of their very low molecular weights. Besides, said methods of synthesis do not allow the preparation of products of higher molecular masses.